It has heretofore been well known in the field of manufacture of silver halide photographic emulsions to add a sensitizing dye to a silver halide emulsion for the purpose of extension of the sensitive wavelength range of the emulsion, i.e., for spectral sensitization thereof. In particular, spectral sensitization in a red-color wavelength range is important in a technique for imparting the necessary red-sensitivity to photographic materials for camera, positive photographic materials for contact papers or enlarging papers, or photographic materials for prints or in a technique for manufacture of photographic materials for printing or recording where a diode, laser, or a cathode ray tube (CRT) capable of emitting a light in a red-color wavelength range is used as a light source. Some kinds of benzimidazolo-dicarbocyanine dyes are known which may be used for the purpose of increasing the red-sensitivity of a silver halide emulsion. Examples of such dyes are described, for example, in Japanese Patent Publication Nos. 22883/68, 42495/73 and 25331/77; Japanese Patent Application (OPI) Nos. 24726/73, 4822/77 and 151026/77 (the term "OPI" as used herein refers to a "published unexamined Japanese patent application"); U.S. Pat. Nos. 3,264,110, 3,431,110, etc. However, the sensitivity is low when the benzimidazolo-dicarbocyanine dyes as described in the above patent specifications are used, and none of them has provided sufficiently desirable results. In addition, the sensitivity of these dyes lowers with the lapse of time when preserved for a long period of time, which is another problem.
In the recent years, an electronic color separation scanner with a laser ray-light source has been used in many cases in the field of printing. In particular, a neon/helium laser having a power wavelength of 632.8 nm is used in most cases. As for the spectral sensitization technique for silver halide photographic materials for formation of dot images by such a scanner-type high illumination and short period exposure, rhodacyanine dyes as described, for example, in Japanese Patent Application (OPI) Nos. 62425/75, 18726/79, and 151933/81 are known. However, some other improved spectral sensitization method is desired, which may attain a higher sensitivity to the neon/helium laser ray and which is free from any color stain due to the remaining dyes after development and fixation.